1. Field of the Invention
This invention concerns a thermal oil recovery method utilizing steam and in-situ combustion in a method which permits efficient recovery from formations with continuously underlying water zones.
2. Description of the Prior Art
In the recovery of petroleum crude oils from subterranean reservoirs, it usually is possible to recover only a minor portion of the oil originally in place in a reservoir by the so-called primary recovery methods, i.e., those methods which utilize only the natural forces present in the reservoir. Thus, a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. In these supplemental techniques which are commonly referred to as secondary recovery operations, although they may be primary or tertiary in sequence of employment, energy is supplied to the reservoir as a means of moving the oil in the reservoir to suitable production wells through which it may be withdrawn to the surface of the earth. Perhaps the most common secondary recovery processes are those in which displacing fluids such as water or gas are injected into an oil-bearing reservoir in order to displace the oil therein to suitable production wells.
Steam has been used in many different methods for the recovery of oil from subterranean, viscous oil-containing formations. The two most basic processes using steam for the recovery of oil includes a "steam drive" process and a "huff and puff" steam process. Steam drive involves injecting steam through an injection well into a formation. Upon entering the formation, the heat transferred to the formation by the steam lowers the viscosity of the formation oil, thereby improving its mobility. In addition, the continued injection of the steam provides the drive to displace the oil toward a production well from which it is produced. Huff and puff involves injecting steam into a formation through an injection well, stopping the injection of steam, permitting the formation to soak and then back producing oil through the original injection well.
Another secondary recovery process which has shown promise is the concurrent or forward burn in-situ combustion technique. In this procedure, a portion of the reservoir oil is burned or oxidized in-situ to create a combustion front. This combustion front is advanced through the reservoir in the direction of one or more production wells by the injection of a combustion-supporting gas through one or more injection wells. The combustion front is preceded by a high temperature zone, commonly called a "retort zone," within which the reservoir oil is heated to effect a viscosity reduction and is subjected to distillation and cracking. Hydrocarbon fluids including the heated, relatively low viscosity oil and the distillation and cracking products of the oil then are displaced toward production wells where they are subsequently withdrawn to the surface of the earth. The in-situ combustion procedure is particularly useful in the recovery of thick, heavy oils such as viscous petroleum crude oils and the heavy, tar-like hydrocarbons present in tar sands. While these tar-like hydrocarbons may exist as solid or semi-solid materials in their native state, they undergo a sharp viscosity reduction upon heating and in the portion of the reservoir where the temperature has been increased by the in-situ combustion process behave like the more conventional petroleum crude oils.
Thermal recovery from heavy oil reservoirs underlain by water has been plagued by the poor performance of the Cyclic steam process. Even though the wells are completed in the oil leg only, steam finds a path to the water leg either from behind the casing or by sweeping a small area around the wellbore in the first cycle and only penetrates the water leg in the subsequent cycles. The present invention provides a new and novel recovery technique utilizing steam injection and in-situ combustion for recovering heavy oil from heavy oil-containing reservoir overlying a water saturated zone.